Search

Thermal stimulation is a reservoir enhancement technique that relies on the generation of permeable fractures by thermal gradients. Replication of thermal stimulation in a controlled laboratory environment provides insight to the physical changes that may be expected at the field scale and assist in stimulation optimization. We present a new methodology designed to replicate thermal stimulation in such an environment. We developed an apparatus to allow rocks to be heated to 375 °C at up to 35 MPa confining pressure and rapidly quenched with cold water to ambient temperature while maintaining system pressure. To test our methodology, we characterized three rocks that represent a range of geothermal developments and laboratory based investigations: Lanhélin Granodiorite (granite) from Brittany, France, Allandale Rhyolite from the Lyttelton Volcano, New Zealand and Halswell Basalt also from the Lyttelton Volcano. Our characterization consists of profiling microstructure, porosity, density, dynamic elastic moduli and permeability prior to thermal stressing. Results indicate that we have increased porosity, decreased density, attenuated elastic moduli and significantly enhanced the permeability of the rocks. Rapidly cooled and slowly cooled samples are presented with the rapidly cooled samples showing an increase in permeability. The results are compared to similar studies previously carried out under dry conditions and implications discussed for geothermal operations. [ABSTRACT FROM AUTHOR]

Knowledge of the strength and elastic modulus of a reservoir rock is important for the optimisation of a particular geothermal resource. The reservoir rock for many geothermal projects in the Upper Rhine Graben, such as those at Soultz-sous-Forêts and Rittershoffen (both France), is porphyritic granite. High fracture densities (up to ~ 30 fractures/m) in this reservoir rock require that we consider the strength and elastic modulus of the rock mass, rather than the intact rock. Here we use uniaxial and triaxial deformation experiments performed on intact rock coupled with Geological Strength Index assessments—using the wealth of information from core and borehole analyses—to provide rock mass strength and elastic modulus estimates for the granite reservoir at Soultz-sous-Forêts (from a depth of 1400 to 2200 m) using the generalised Hoek-Brown failure criterion. The average uniaxial compressive strength and elastic modulus of the intact granite are 140 MPa (this study) and 40 GPa (data from this study and the literature), respectively. The modelled strength of the intact granite is 360 MPa at a depth of 1400 m and increases to 455 MPa at 2200 m (using our estimate for the empirical mi term of 30, determined using triaxial and tensile strength measurements on the intact granite). Strength of the rock mass varies in accordance with the fracture density and the extent and nature of the fracture infill, reaching lows of ~ 40-50 MPa (in, for example, the densely fractured zones in EPS-1 at depths of ~ 1650 and ~ 2160 m, respectively) and highs of above 400 MPa (in, for example, the largely unfractured zone at a depth of ~ 1940-2040 m). Variations in rock mass elastic modulus are qualitatively similar (values vary from 1 to 2 GPa up to the elastic modulus of the intact rock, 40 GPa). Our study highlights that macrofractures and joints reduce rock mass strength and should be considered when assessing the rock mass for well stability and rock mass deformation due to stress redistribution in the reservoir. We present a case study to demonstrate how a simple and cost-effective engineering method can be used to provide an indication of the in situ strength and elastic modulus of reservoir rock masses, important for a wide range of modelling and stimulation strategies. We recommend that the effect of macrofractures on rock mass strength and stiffness be validated for incorporation into geomechanical characterisation for geothermal reservoirs worldwide. [ABSTRACT FROM AUTHOR]